If no new CO2-emitting power plants, cars, and other energy and transportation infrastructure were built starting today, Earth might narrowly avoid the worst effects of anticipated global climate change, according to a study.

But that scenario is improbable, say Steven J. Davis of the Carnegie Institution of Washington and colleagues, who prepared the study, because the world is in no position to make the immediate transition to carbon-neutral energy technologies it would require.

Davis and coworkers compiled data on power plant emissions, motor vehicle emissions, and emissions produced directly from industry, households, businesses, and transportation. They then used a climate model to project the effect of future CO2 on Earth’s climate (Science2010,329, 1330).

What the team found surprised them: Even if no new CO2-emitting sources were built, the world’s existing energy infrastructure would emit 500 gigatons of CO2 until current sources go out of service over the next 50 years. That amount would stabilize atmospheric CO2 levels below 430 ppm and level off the average global temperature at 1.3 °C above the preindustrial mean. The researchers had expected those figures to be above the threshold values of 450 ppm and 2 °C that climate scientists believe will trigger major climate disruption.

But there’s still a catch, Davis says. Although existing infrastructure doesn’t appear to be a threat to climate, much of future energy demand will be met by traditional CO2-emitting sources. “The devices whose emissions will cause the worst impacts have yet to be built,” he adds. It will require “truly extraordinary development” of new infrastructure and take decades to distance ourselves from CO2-emitting technologies.

“Efforts to curb emissions through regulation and international agreement haven’t worked, emissions are rising faster than ever, and programs to scale up carbon-neutral energy sources are moving slowly at best,” global environmental change expert Martin I. Hoffert of New York University says in a commentary about the study. “Davis and coworkers offer new insights into just how difficult it will be to say farewell to fossil fuels.”